Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
A typical cellular wireless network includes a number of base stations each radiating to define a respective coverage area in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Further, a cellular wireless network may operate in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Wireless Interoperability for Microwave Access (WiMAX), and Global System for Mobile Communications (GSM), among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover of between coverage areas, and other functions related to air interface communication.
In accordance with the air interface protocol, each coverage area may operate on one or more carrier frequencies or ranges of carrier frequencies. Further, each coverage area may define a number of channels or specific resources for carrying signals and information between the base station and UEs. For instance, certain resources on the downlink may be reserved to carry a pilot or reference signal that UEs may detect as an indication of coverage and may measure to evaluate coverage quality. Further, certain resources on the uplink may be reserved to carry access requests from UEs seeking to gain access to the base station. And certain resources on the downlink may be reserved to carry control messaging such as paging messages and random access response messages from the base station. In addition, certain resources on the uplink and downlink may be set aside to carry bearer traffic (e.g., user communications) in a manner assigned or scheduled by the base station for instance.
When a UE is within coverage of a base station, the base station may from time to time transmit downlink control signaling to the UE. The purpose of such downlink control signaling may depend on the air interface protocol and the circumstances. By way of example, each coverage area may define a downlink control channel that may carry certain information such as control channel formatting information, a downlink reference signal that UEs may detect as an indication of coverage, system information, paging information, and the like, or may define various downlink control channels for carrying subsets of this information.
In general, a UE may operate in a particular coverage area provided by a base station by transmitting to the base station an “attach” request or the like to register with the base station and trigger reservation of network resources for the UE, and then operating in a connected mode or an idle mode. In the connected mode, the UE may have assigned air interface resources defining an air interface “connection,” and the UE and the base station may be set to exchange bearer data with each other, with the base station possibly providing downlink control signaling to the UE to assign specific air interface resources on which the bearer data communication is to occur. After a timeout period of no bearer data communication between the UE and the base station, or for other reasons, the UE may then transition from the connected mode to the idle mode, with the base station releasing the UE's air interface connection so as to conserve air interface resources. In the idle mode, the UE may then monitor a downlink control channel to receive overhead system information and to check for any page messages destined to the UE. If the UE then receives a page message to which the UE will respond and/or if the UE seeks to engage in bearer communication, the UE may then transmit on an uplink control channel to the base station a random access preamble or other such request, to which the base station may respond on a downlink control channel, and the UE may transition back to the connected mode.
A UE may also move between neighboring coverage areas of one or more base stations. More specifically, as a UE moves between wireless coverage areas of a wireless communication system, such as between different base stations, or when network conditions change or for other reasons, the UE may “hand off” or “handover” from operating in one coverage area to operating in another coverage area. In a usual case, this handover process is triggered by the UE monitoring the signal strength of various nearby available coverage areas, either as a matter of routine operation, or in response to a directive by the UE's serving base station to monitor for one or another handover trigger condition. When a handover condition is met, the UE's serving base station may then initiate a handover procedure. By convention, a UE is said to handover from a “source” (or serving) base station to a “target” base station.
In a further aspect of LTE, Under LTE, downlink and uplink air interface resources are mapped into frames, with each frame being further divided into a number of subframes that are further divided into slots. LTE further defines a particular grouping of resources arrayed across one subframe in the time-domain and 12 sub-carriers in the frequency-domain as a “resource block.” Further, each resource block contains an array of 14 OFDM symbols by 12 sub-carriers, with each symbol and sub-carrier pair being referred to as a “resource element.” A resource block may therefore include 168 “resource elements.”
An eNodeB typically broadcasts a reference signal in a number of resource elements distributed throughout the downlink frequency bandwidth. More specifically, there may be particular resource elements that are designated for resource signals in each subframe that is transmitted in a given coverage area. A UE operating in the coverage area may receive these reference signals, and further, may determine the received power level of each reference signal that is received. The UE may then determine a measure of the eNodeB's signal strength, which is referred to as a reference signal received power (RSRP). In particular, the UE may determine the RSRP by determining an average received power across all the reference signals that are received in the same subframe.
In some cases, when the UE is operating in a first coverage area, the UE may determine an RSRP a second coverage area that ultimately triggers a handover of the UE to the second coverage area. More specifically, a UE may report the RSRP for a second coverage area to the eNodeB that is serving the UE in a first coverage area. This eNodeB may in turn use the RSRP to evaluate whether the UE should handoff to the second coverage area; e.g., by comparing the RSRP for the second coverage area to a measure of signal strength in the first coverage area. Then, if the eNodeB determines a handoff is appropriate, the eNodeB may instruct the UE to handoff to the second coverage area.